1. Field of the Invention
This invention relates to a Group III nitride semiconductor light-emitting device, and more particularly to a method of forming single crystal films for use in the light-emitting device.
2. Description of the Related Art
In the manufacturing of a light-emitting device using semiconductors, such as a light-emitting diode (LED) and a laser diode (LD), semiconductor layers with varied forbidden band width (hereinafter simply referred to as "band gap" or "E.sub.g ") are formed one upon another to form a basic structure of the device. In a Group III nitride semiconductor light-emitting device according to the present invention, x and y values of (Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y N (0.ltoreq.x1, 0.ltoreq.y.ltoreq.1) are varied to thereby change values of the band gap.
FIG. 1 shows an example of the basic structure of a laser diode device using Group III nitride semiconductors, in which a GaN or AIN layer 2 is formed on a single crystal sapphire substrate 1 at a low temperature, and then on the layer 2, an n-type GaN layer 3, an n-type Al.sub.0.1 Ga.sub.0.9 N layer 4, an n-type GaN layer 5, an active layer 6 having InGaN as a major constituent thereof, a p-type GaN layer 7, a p-type Al.sub.0.1 Ga.sub.0.9 N layer 8, and a p-type GaN layer 9 are formed one upon another in the mentioned order. An n-type electrode 101 and a p-type electrode 102 are formed on the n-type GaN layer 3 and the p-type GaN layer 9, respectively.
In this laser diode device constructed as above, light is emitted by electron-hole recombination in the active layer 6. The n-type GaN layer 5 and the p-type GaN layer 7 are guide layers, within which light generated in the active layer 6 is waveguided. Further, it is possible to confine electrons and holes within the active layer 6 effectively by setting the band gap of each of the guide layers to be larger than that of the active layer 6. The n-type Al.sub.0.1 Ga.sub.0.9 N layer 4 and the p-type Al.sub.0.1 Ga.sub.0.9 N layer 8 are clad layers having a refractive index which is lower than that of the p-type GaN layer 7, and the aforementioned waveguiding of the light is effected by the difference between the refractive index of the clad layers and that of the guide layers.
The n-type GaN layer 3 is an underlying layer which provides a current path because the sapphire substrate has no conductivity. Further, the low-temperature growth layer 2, so-called a buffer layer, is formed for producing a smooth GaN film on the sapphire substrate which is a dissimilar material to GaN.
In the Group III nitride semiconductors (Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y N (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1), considering GaN is a basic binary compound, it is possible to replace part of the Ga atoms with Al atoms by doping the compound with Al to thereby increase the band gap of the same. Further, it is possible to replace part of the Ga atoms with In atoms by doping the compound with In to thereby decrease the band of the same. As the value of the band gap becomes larger, the refractive index is reduced.
In an Al.sub.z Ga.sub.1-z As/GaAs system used in a laser diode operative in the infrared region, a lattice constant thereof hardly changes irrespective of the z value, whereas in the Group III nitride semiconductors (Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y N (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1), if the x and y values are varied, a lattice constant thereof changes sharply. This difference results from the fact that in the case of Al.sub.z Ga.sub.1-z,As system, the lattice constant of GaAs and that of AlAs are approximately equal to each other, so that no lattice mismatch occurs.
In the manufacturing of the semiconductor device described above by using Group III nitride semiconductors (Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y N (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1), cracking (or crazing) occurs during formation of the n-type AlGaN layer 4. The lattice constant of AlN is smaller than that of GaN by approximately 2.4% in a-axis direction, so that when the AlGaN-clad layer 4 is formed on the GaN underlying layer 3, tensile stress is generated paralell to the interface between the two layers 3 and 4. In general, a semiconductor crystal is resistant to compressive stress, but brittle to tensile stress. For this reason, cracking occurs very easily in the AlGaN-clad layer 4.
This cracking in the AlGaN-clad layer 4 propagates through the underlying layer 3 as well as through the guide layer 5 formed on the AlGaN-clad layer 4. A laser diode is a device which is operated by guiding optical waves (i.e. by waveguiding light) within a layered structure thereof. Therefore, the cracking can fatally affect the characteristics of the device.